Method for surface hardening steel and cemented carbides

ABSTRACT

A method of surface hardening steel and cemented carbides, wherein the material to be treated is at a temperature exceeding approximately 570* C and while extensively excluding oxygen is brought into contact with a mixture containing approximately 5 70 parts by weight borium carbide, 1 - 60 parts by weight anhydrous aluminum oxide, trace amounts up to 6 parts by weight NaBF4, and trace amounts up to 10 parts by weight sodium fluoride.

United States Patent 1191 METHOD FOR SURFACE HARDENING STEEL AND CEMENTED CARBIDES [76] inventor: Anton Bopp, Dorfstrasse 198,

Meilen, Switzerland [22] Filed: July 26, 1971 [21] App]. No.: 165,948

[30] Foreign Application Priority Data 1 Nov. 6, 1973 3,286,684 12/1962 Aves 117/1072 P 3,622,374 11/1971 Pike 3,029,162 4/1962 Samuel et al 117/107.2 P

[57] ABSTRACT A method of surface hardening steel and cemented ear- July 28, 1970 Switzerland 11484/70 bides, wherein the material to be treated 1s at a temper- 52 US. Cl 148/6, 1 17/1072, 148/63 alum/exceeding pp m y 7 C and while eaten- 1] Int CL C23f /00, C23f 7/04, C23c 9/04 sively excluding oxygen 15 brought into contact with a 581 Field of Search 148/6, 6.3; mixture wmaimng approximately 5 Parts y 117/1072 p, 13 weight borium carbide, 1 parts by weight anhydrous aluminum oxide, trace amounts up to 6 parts by [56] References Cited weight NaBF,,, and trace amounts up; to 10 parts by UNITED STATES PATENTS weigh sdium 3,673,0()5 6/1972 Kunst 148/6 21 Claims, 10 Drawing Figures x 2000.. i Q i 7600 480 =1.

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METHOD FOR SURFACE HARDENING STEEL AND CEMENTED CARBIDES BACKGROUND OF THE INVENTION The present invention relates to a new and improved method for surface hardening steel and cemented carbides wherein, owing to a thermal treatment in a'mixture, there'is obtained at the material-undergoing treatment a relatively thick, fixedly anchored, infusion layer of high hardness.

It is well known to the art to harden steel by carburizing, nitriding and boriding. During the boriding of steel boron diffuses into the steel while forming thin, hard, wear resistant iron boride layers anchored in tooth-like fashion at the steel base or matrix. Duringsolid phase boriding there is employed amorphous boron apart from ferrous boron and/or B,,C in fine grain size together with A1 and an activator, for instance NI-I Cl. However, when using this technique only relatively thin films of about 0.6 millimeters thickness are obtained if treatment is undertaken at sealed carburization boxes or ceramic crucibles at temperatures of 950 C to 1,050 C for 5 to 25 hours. When working with ethyl silicate paste containing the same parts by weight B C and cryolite in ahigh frequency induction furnace at approximately l,200 C there are'obtained infusion layers up to 0.125 millimeters thickness within approxi mately 3 minutes. What is disadvantageous with this technique is that there appears a variable film thickness and the possible welding of particles.

Salt bath boriding with sodium chloride, barium chloride and boron carbide or sodium chloride, sodium borofluoride and boron carbide work at temperatures between approximately 900 C and l,000 C. The practical times employed last between 5 to hours. There can be thus separated out boron carbide. It has been found that the boron trifluoride which appears only is present as activator and the effective component is only the boron carbide.

Furthermore, it is known to perform the boriding of steel cathodically in a borax melt with the addition of activators. Owing to the relatively high viscosity of the bath melt the oftentimes operating procedures only can be poorly regulated. What is also disadvantageous is the formation of crusts which cannot be easily removed.

It is also known to the art to carry out boriding in hydrogen in gaseous phase with BCl and B l-I yet these procedures are only usable to a very limited extent owing to their high cost and the required expenditure of material.

SUMMARY THE INVENTION Hence, from what has been stated above-it will be seenthat the prior art is still in need of an improved method of surface hardening steel and sintered or cemented carbides in a manner not associated with the aforementioned drawbacks. It is aprimary objective of this inventionto provide just such improved method of surface hardening steel and cemented carbides in a most economical, reliable and relatively simple fashion.

It is another object of this invention to provide a novel method of surface hardening materials, such as steel and cemented carbides, in a manner where the treatment can be carried out extremely simply, at relatively low temperatures, resulting in the formation of relatively thick, fixedly anchored infusion layers possessing a hardness previously not attained.

Now, in order to implement these and still further objects of the invention, which will become more readily apparent as the description proceeds, the inventive method for surface hardening of materials, especially steel and cemented carbides is generally manifested by the features that, the material, at a temperature exceeding approximately 570 C and extensively in the absence of oxygen, is brought into contact with a mixture containing approximately:

5 parts by weight boron carbide,

l 60 parts by weight anhydrous aluminum oxide,

Trace amounts 6 parts by weight NaBF Trace amounts 10 parts by weight sodium fluoride.

The method can be used for the most different steels, such as carbon steels, alloyed and highly-alloyed steels, specialty steels, also cemented carbides and pressed cemented carbides and works with prepared powdery materials in accordance with the case-hardening principle. The exceptional action of the previously mentioned mixture is predicated upon the fact that the components are present in a glassy or vitreous condition, so that ionic excitations occur which result, for instance, in reactions of the following occur which result, for instance, in reactions of the following nature:

Therefore, it can be assumed there is formed tetrafluoromethane which can be effective as activator and solvent for oxides and hydroxides.

The carbon necessary for this purpose can be either additionally added, especially in the form of carbon black, or it can be removed from the carbon content of the steel. Furthermore, if desired, tetrafluoromethane can be added separately to the mixture in the form of a gas. I 7

Furthermore, the mixture used in the practice of the inventive method, in accordance with the momentary intended use, has added thereto additional materials known to some extent for this purpose. Thus, for instance, the addition of magnesium in the form of basic magnesium carbonate prevents any desired sintering of the hard metal or cemented carbide composition and permits a lowering of the temperatures, beneath 600 C, for instance. Suitable as the additives are, in particular, Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, B, as such, or their oxides, carbides, borides, nitrides or silicides.

The components are used in a powdery form, a grain or granulation size of approximately 180 to 320 mesh being preferred. In the event that in special circumstances very voluminous or bulky objects are treated, then, the powdery mixture is dispersed in a liquid carrier, such as in particular dibutyl phthalate or an oil, so that it can be conveniently applied to the surface of the object or body.

The treatment temperature is dependent upon the type of steel to be treated, generally is in a range above 570 C up to white heat or incandescence. The best steel hardness has been obtained at temperatures between approximately 750 C and 900 C, only conventional high temperature hardners, such as certain stainless steels (SS) and hot forged steels which can core soften between 750 C and 900 C require lower temperatures. Towards the upper range the temperatures can be increased to approximately 1, 1 50 C. Treatment temperatures in the region of approximately 850 C are quite suitable.

The inventive method is carried out extensively in the absence of oxygen, that is, air is excluded, or in the presence of a protective gas, such as nitrogen, inert or rare gases or CO Performance of the method is exceptionally simple: the clean steel product to be hardened is embedded in the powder, within a heatand corrosion resistant container or vessel, for instance formed of chromiumnickel-steel, graphite or the like, in such a manner that all sides of such product are tightly or snugly in contact with this powder. The vessel is then hermetically sealed, if desired oxygen is removed, and in a suitable and well known manner electrically heated, or heated in a reverberatory or air furnace up to the required temperature. Even after very short treatment times, for instance between 3 to 5 hours, exceptional hardness results have been obtained.

After the completion of the hardening operation the steel product together with the vessel is advantageously air cooled down to approximately 500 C. At this temperature the strewable powder can be removed through filters or sieves without suffering from contact with air. The steel product, if required, can be easily freed of any adhering powder. The powder then can be reused.

A particularly preferred mixture, used for the following trials, possesses the following composition:

8 C Industrial anhydrous aluminum oxide 50 parts by weight 43 parts by weight In FIGS. 1 to 10 there are indicated surface hardness, core hardness and layer thickness of the infusion layer as a function of the employed temperature for different types of steels and cemented carbides. The test samples have been pre-treated by the manufacturer, in the manner indicated in the following table, whereupon such have been treated in accordance with the inventive method as above-described at temperatures between 750 C and 900 C and permitted to cool in the furnace. The chain-dot or phantom line curve in each case indicates the results for a treatment time of 5 hours, the full-line curve for a treatment time of hours.

Cr 4.39, Ni 0.20, W 0.34, 510C 2 X 1 hour air V 1.87, Mo 4.87 3 C l.ll,Si 0.16, Mn 0.17,

Cr 0.14, Ni 0.10 770C 10' water 200C 20 air 4 C 2.06, Si 0.42, Mn 0.35, 960C 15 oil Cr 13.27, Ni 0.17, W 0.37 420C 30' air 5 C 0.09, Si 0.48, Mn 0.13, 990C 10 oil Cr 13.94, Ni 0.25 580C 1 hour air (305 I-IV 6 C (3-046, Si Mn 2 1040C 10' water (243 HVOJ) Cr 19.0, Ni 9.92 7 C 0.013, Si 0.06, Mn 0.10, 820C 1 hour air Cr 0.1 1, Ni 18.40, Mo 5.00, 470C 4 hours air (551 I-IV Co 7.88, Ti 0.43, A1012 8 C 0.52, Si 1.45, Mn 0.67, 1050C 30 air Cr 4.28, No 12.39, W 10.53, 760 C 16 hours air (446 HV D V 0.92, Cu 1.61 9 C 0.04, Si 0.08 Mn 1.30 980C 30' air Cr 15.25, Ni 26.80, V 0.40, 725C 16 hours air (375 HV B 0.0037, M0 1.28, Al

Tl WC 94%, Co 6% 13 C Industrial anhydrous aluminum oxide 85 parts by weight 73 parts by weight Carbon black 3.5 parts by weight NaBF 3.5 parts by weight NaF 5 parts by weight 25 parts by weight Basic magnesium carbonate 365 parts by weight Dibutyl phthalate The solid components are dispersed in the liquid carrier and the dispersion is applied by spraying, by immersion, or by wiping such onto the material or product to be treated.

The advantages of the inventive method are considerable. At relatively low temperatures and short treatment times it is possible to obtain hardness values which previously could not be attained, values which oftentimes exceed the peak values of extremely hard sintered materials. The mixture of the invention may contain 1 to 10 ppm NaBF and l to 10 ppm sodium fluoride (NaF).

Owing to the low treatment temperature there are hardly any volume changes or any appreciable volume changes of a disturbing nature. The employed powder, following treatment, remains completely strewable or pourable, can be used a number of times, sometimes as much as 5 to 10 times. The consumption of material is minimum, since it is limited to 0.02% to 0.8% of the weight of the infusion layer. Carrying out of the inventive method is extremely simple and it has versatile ap plicability. The treated material remains clean, dark in color and previously polished surfaces exhibit a dull gloss.

While there is shown and described present preferred embodiments of the invention, it is to be distinctly unders'tood that the invention is not limited thereto but may be otherwise variously embodied and practiced within the scope of the following claims. Accordingly,

What is claimed is:

1. A method of surface hardening materials by a boriding process, especially steel and cemented carbides, comprising the steps of placing the material which is at a temperature in a range exceeding approximately 570 C to approximately 900 C and essentially in the absence of oxygen, into contact with a mixture consisting essentially of 5 to parts by weight boron carbide, 1 to 6 parts by weight anhydrous aluminum oxide, trace amounts up to 6 parts by weight N'aBF, and an amount of sodium fluoride up to 10 parts by weight sufficient to effect reaction at the material temperature.

2. The method as defined in claim 1, further including the step of adding tetrafluoromethane.

3. The method as defined in claim 1, further including the step of adding to the mixture basic magnesium carbonate.

4. The method as defined in claim 1, further including the step of adding to the mixture carbon.

' 5. The method as defined in claim 1, further including the step of adding to the mixture a member selected from the group consisting of Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, B, or the oxides, carbides, borides, nitrides or silicides thereof.

6. The method as defined in claim 1, wherein the components of the mixture are present in a powdery form with a granulation size of approximately 180 to 320 mesh with said surface of said material to be hardened being compacted in said powder.

7. The method as defined in claim 1, further including the step of adding a liquid carrier to the mixture.

8. The method as defined in claim 1, of adding dibutyl phthalate as a liquid carrier.

9. The method as defined in claim 1, wherein the mixture provided is a powder having the following composition:

B,C Industrial anhydrous aluminum oxide 50 parts by weight 43 parts by weight Carbon black 2 parts by weight NaBF 2 parts by weight NaF 3 parts by weight 10. The method as defined in claim 7, wherein the mixture provided is a liquid form having the following composition:

ac Industrial anhydrous aluminum oxide 85 parts by weight 73 parts by weight Carbon black 3.5 parts by weight NaBF 3.5 parts by weight NaF 5 parts by weight Basic ma nesium carbonate Dibutyl p thalate 25 arts by weight 365 parts by weight l,l50 C.

13. The method as defined in claim 1, including the step of carrying out treatment at a temperature around 850 C.

14. The method as defined in claim 1, including the step of carrying out treatment at a temperature in the range of about 750 C to 900 C.

15. The method as defined in claim 1, wherein the initial step consists of embedding the material undergoing treatment in the mixture of powdery form.

16. The method as defined in claim 1 wherein the initial step consists of applying a dispersion of said mixture in a liquid carrier to the material undergoing treatment.

17. The method as defined in claim 16, wherein the step of applying the dispersion in the liquid carrier to the material undergoing treatment is by spraying such thereon.

18. The method as defined in claim 17, wherein the step of applying the dispersion which is in a liquid carrier to the material undergoing treatment is by immersing such material therein.

19. The method as defined in claim 16, wherein the step of applying the dispersion in a liquid carrier to the material undergoing treatment is by wiping such thereon.

20. The method as defined in claim 1, including the step of providing for the exclusion of air for carrying out the treatment.

21. A composition of matter for use in surface hardening materials, especially steel and cemented carbides, said composition of matter consisting essentially of a mixture containing approximately 5 to by weight boron carbide, l to 60% by weight anhydrous aluminum oxide, trace amounts up to 6% by weight NaBF and trace amounts up to 10% by weight sodium fluoride. 

2. The method as defined in claim 1, further including the step of adding tetrafluoromethane.
 3. The method as defined in claim 1, further including the step of adding to the mixture basic magnesium carbonate.
 4. The method as defined in claim 1, further including the step of adding to the mixture carbon.
 5. The method as defined in claim 1, further including the step of adding to the mixture a member selected from the grouP consisting of Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, B, or the oxides, carbides, borides, nitrides or silicides thereof.
 6. The method as defined in claim 1, wherein the components of the mixture are present in a powdery form with a granulation size of approximately 180 to 320 mesh with said surface of said material to be hardened being compacted in said powder.
 7. The method as defined in claim 1, further including the step of adding a liquid carrier to the mixture.
 8. The method as defined in claim 1, of adding dibutyl phthalate as a liquid carrier.
 9. The method as defined in claim 1, wherein the mixture provided is a powder having the following composition: B4C 50 parts by weight Industrial anhydrous aluminum oxide 43 parts by weight Carbon black 2 parts by weight NaBF4 2 parts by weight NaF 3 parts by weight
 10. The method as defined in claim 7, wherein the mixture provided is a liquid form having the following composition: B4C 85 parts by weight Industrial anhydrous aluminum oxide 73 parts by weight Carbon black 3.5 parts by weight NaBF4 3.5 parts by weight NaF 5 parts by weight Basic magnesium carbonate 25 parts by weight Dibutyl phthalate 365 parts by weight
 11. The method as defined in claim 1, wherein the mixture contains 1 to 10 ppm NaBF4 and 1 to 10 ppm sodium fluoride.
 12. The method as defined in claim 1, including the step of carrying out treatment at a temperature up to 1,150* C.
 13. The method as defined in claim 1, including the step of carrying out treatment at a temperature around 850* C.
 14. The method as defined in claim 1, including the step of carrying out treatment at a temperature in the range of about 750* C to 900* C.
 15. The method as defined in claim 1, wherein the initial step consists of embedding the material undergoing treatment in the mixture of powdery form.
 16. The method as defined in claim 1 wherein the initial step consists of applying a dispersion of said mixture in a liquid carrier to the material undergoing treatment.
 17. The method as defined in claim 16, wherein the step of applying the dispersion in the liquid carrier to the material undergoing treatment is by spraying such thereon.
 18. The method as defined in claim 17, wherein the step of applying the dispersion which is in a liquid carrier to the material undergoing treatment is by immersing such material therein.
 19. The method as defined in claim 16, wherein the step of applying the dispersion in a liquid carrier to the material undergoing treatment is by wiping such thereon.
 20. The method as defined in claim 1, including the step of providing for the exclusion of air for carrying out the treatment.
 21. A composition of matter for use in surface hardening materials, especially steel and cemented carbides, said composition of matter consisting essentially of a mixture containing approximately 5 to 70% by weight boron carbide, 1 to 60% by weight anhydrous aluminum oxide, trace amounts up to 6% by weight NaBF4, and trace amounts up to 10% by weight sodium fluoride. 